With the progress of telecommunication technology, radio communication modules are carried on various devices and systems such as various mobile communication devices, ISDN (integrated service digital network) and computer devices, and enable high-speed communication of data information and the like. The radio communication modules are reduced in size and weight, combined, or made multifunctional. In high-frequency applications using microwaves and millimeter waves as carrier frequencies, for example, in a communication device constituting a radio LAN (local area network) or the like, the radio communication modules cannot achieve the above-described specification requirements in a circuit based on a concentrated constant design in which a low-pass filter, a high-pass filter, a band-pass filter, a coupler and the like use chip components such as capacitors and coils, and a distributed constant design using microstrip lines, strip lines and the like is generally is used.
Conventionally, a band-pass filter (BPF) 100 based on a distributed constant design is formed by cascading plural resonator conductor patterns 102a to 102e on a major surface of a dielectric board 101, for example, as shown in FIG. 1. In the BPF 100, a high-frequency signal is inputted from the first outer conductor pattern 102a, and a high-frequency signal of a predetermined frequency band is selected by the second to fourth conductor patterns 102b to 102d arranged on the inner side and outputted from the fifth outer conductor pattern 102e. Except for the conductor pattern 102c at the central part, the conductor patterns 102 are coupled on the lateral side of the board 101. Although not shown, a ground pattern is formed on the entire rear side of the board 101.
In the BPF 100, the conductor patterns 102a to 102e adjacent to each other cascaded on the major surface of the dielectric board 101 as described above in such a manner that they overlap each other within a range of length of ¼ of the passing wavelength λ, as shown in FIG. 1. Since the conductor patterns 102 are formed on the board 101 having a high dielectric constant, the length of each conductor pattern 102 can be reduced by the wavelength shortening effect of the microstrip line and the BPF 100 can be miniaturized.
The shortening of wavelength occurs at λ0/√∈w (where λ0 represents the wavelength in vacuum, and ∈w represents the effective relative dielectric constant, which is determined by electromagnetic field distribution of air and dielectric material) on the outer layer of the board 101, and also occurs at λ0/√∈r (where ∈r represents the relative dielectric constant of the board). Therefore, the BPF 100 selectively transmits a high-frequency signal of a desired frequency band by optimizing the conductor patterns 102a to 102e. In the BPF 100, since the conductor patterns 102 can be formed by on the major surface of the board 101 by performing printing or lithography processing as in a general wiring board forming process, these can be formed simultaneously with circuit patterns.
Even in such a BPF 100, the length of each of the conductor patterns 102a to 102e is regulated by the passing wavelength λ because the conductor patterns 102a to 102e overlap each other with an overlapping length substantially equal to λ/4 of the passing wavelength as they are arrayed. Therefore, the board 101 of a certain size is necessary to cover the lengths of the conductor patterns 102a to 102e, and the miniaturization of the BPF 100 is limited.
Meanwhile, another conventional BPF 110 shown in FIGS. 2A to 2C and FIG. 3 is formed by a so-called triplate structure in which resonator conductor patterns 113, 114 are formed within a multilayer board including a pair of dielectric boards 111, 112. Ground patterns 115, 116 are formed on the surfaces of the dielectric boards 111, 112, respectively, as shown in FIGS. 2A and 2C. Multiple via-holes 117 are formed in outer circumferential parts of the dielectric boards 111, 112 and continuity between the ground patterns 115, 116 on both sides is made, thus shielding the inner layer circuit.
Each of the resonator conductor patterns 113, 114 has a length M, which is substantially ¼ of the passing wavelength λ, and the resonator conductor patterns 113, 114 are formed in parallel with their one ends connected to the ground patterns 115, 116 and their other ends opened, as shown in FIG. 2B. On the resonator conductor patterns 113, 114, input/output patterns 118, 119 protruding in an arm-like shape toward the lateral side are formed. In the BPF 110, the resonator conductor patterns 113, 114 formed in the above-described dielectric boards 111, 112 are constructed to have parallel resonance circuits that are capacitive-coupled like equivalent circuits as shown in FIG. 3. Specifically, in the BPF 110, a parallel resonance circuit PR1 formed by a capacitor C1 and an inductor L1 connected between the resonator conductor pattern 113 and the ground pattern, and a parallel resonance circuit PR2 formed by a capacitor C2 and an inductor L2 connected between the resonator conductor pattern 114 and the ground pattern, are capacitive-coupled via a capacitor C3.
Such a BPF 110 has a function of resonating an open line of substantially λ/2 with respect to a high-frequency signal having a wavelength λ, in a predetermined frequency band, and utilizes the face that the degree of coupling reaches the maximum at λ/4. With this BPF 110, a high-frequency signal having a wavelength λ inputted from the resonator conductor pattern 113 is caused to resonate in the bans of the predetermined passing wavelength λ by the parallel resonance circuit PR1 and the parallel resonance circuit PR2. High-frequency components out of the band are removed and the signal is then outputted. The BPF 110 is miniaturized as the lengths of the resonator conductor patterns 113, 114 formed in the dielectric boards 111, 112 are substantially λ/4.
Meanwhile, as the size and weight of mobile communication devices are further reduced, a radio communication module having an overall size of, for example, 10×10 mm or less, is demanded. Particularly in the case of carrying a radio communication module on a consumer mobile communication device or the like that has extremely tight cost requirements, the radio communication module must be equivalent to an inexpensive printed board that is generally used as board material.
The BPF 110 cannot meet the above-described specification requirements though the overall length of the resonator conductor patterns 113, 114 is reduced to λ/4. That is, in a radio LAN system or a short-distance radio transmission system called Bluetooth, the carrier frequency band is regulated to 2.4 GHz and the carrier wavelength λ0/4 in the space is approximately 30 mm. Even if the resonator conductor patterns 113, 114 are built in a copper-clad multilayer board of FR grade 4 having a relative dielectric constant of approximately 4, which is carried on a radio communication module of a mobile communication device conformable to such a system and is generally used as a board material, for example, a copper-clad multilayer board made of burning-resistant glass cloth base epoxy resin, the passing wavelength λ/4 is approximately 15 mm. Therefore, the BPF 110 cannot meet the above-described specification requirements.
It may be considered that, for example, a ceramic material having a relative dielectric constant of 10 or more is used to improve the wavelength shortening effect and thus to miniaturize the BPF 110. Such a BPF 110 needs a large board when integrating peripheral components for a radio communication module, and the cost is increased by the use of the ceramic board, which is relatively expensive. Therefore, the above-described cost requirement cannot be met.
In the above-described BPF 110, the filter characteristics such as passing band characteristic and cutoff characteristic are determined by electromagnetic field distribution between the dielectric boards 111, 112 and between the resonator conductor patterns 113, 114. In the BPF 110, the strength of the electric field changes in accordance with the facing spacing p between the resonator conductor patterns 113, 114 in an odd excitation mode and also changes in accordance with the spacing between the dielectric boards 111, 112 and the resonator conductor patterns 113, 114 in an even excitation mode, that is, the thickness t of the dielectric boards 111, 112 shown in FIG. 2A. In the BPF 110, the strength of the electric field also changed in accordance with the width w of the resonator conductor patterns 113, 114 as shown in FIG. 2A.
In the BPF 110, since the strength of the electric field changes in accordance with the odd excitation mode or even excitation mode, the degree of coupling of the resonator conductor patterns 113, 114 changes and the filter characteristics change. In the BPF 110, the dielectric boards 111, 112 and the resonator conductor patterns 113, 114 are precisely formed in order to realize desired filter characteristics.
Generally, in the BPFs, desired filter characteristics cannot be achieved because of some difference in the manufacturing process, and an adjustment process is performed, for example, based on additional processing for properly changing the position and area of the resonator conductor patterns while checking their output characteristics by a measuring device or the like. In the BPF 110, since the resonator conductor patterns 113, 114 are formed in the inner layer of the dielectric boards 111, 112 as described above, it is difficult to perform such an adjustment process. Therefore, as a highly accurate manufacturing process to produce each part is employed for the BPF 110, the manufacturing efficiency is lowered and also the yield is lowered.